Rotary to linear LVDT system

ABSTRACT

A rotary input for a linear variable differential transformer (LVDT) system connects to a gear reduction system. The gear reduction produces a reduced rotary output in a rotary output member. The output member is threaded, and a nut is threaded onto the output member. Therefore, the nut translates as the rotary output member rotates. The armatures of one or more LVDTs attach to the nut. Each armature contains a magnetic member. As the nut translates along the threaded output member, the nut translates the armatures. Armature movement moves the magnetic members to create a differential voltage between the secondary coils of the LVDT.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device that uses a linear variabledifferential transformer (LVDT) for measuring rotation of a shaft.

2. General Background and State of the Art

A linear variable differential transformer (LVDT) is a displacementtransducer that produces an electrical signal proportional to thedisplacement of a moveable core (armature) within a cylindricaltransformer. The transformer consists of a central, primary coil windingand two secondary coil windings on opposite ends of the primary winding.The coil windings are coaxial. The armature preferably is nickel-ironand is positioned within the coil assembly. The core provides a path formagnetic flux linking the primary coil to the secondary coils.

When the primary coil is energized with an alternating current, acylindrical flux field is produced over the length of the armature. Thisflux field produces a voltage in each of the two secondary coils thatvaries as a function of the armature position. Armature movement movesthe flux field into one secondary and out of the other causing anincrease in the voltage induced in one secondary and a correspondingvoltage decrease in the other. The secondary coils are normallyconnected in series with opposing phase. The net output of the LVDT isthe difference between the two secondary voltages. When the armature ispositioned symmetrically relative to the two secondary windings (the“null” position), the differential output is approximately zero, becausethe voltage of each secondary is equal but of opposite phase.

Subjecting an actuator to pressure or force can move an LVDT armaturethrough a linkage. Thus, LVDTs are commonly used in pressuretransducers. As pressure increases, the armature moves toward onesecondary winding and away from the other. This yields a voltagedifference that can be proportional to the pressure on the transducer.Consequently, this voltage output can measure pressure and position.

Nearly all LVDTs that are designed for aircraft or missile applicationsare wound on an insulated stainless steel spool, magnetically shieldedand enclosed in a stainless steel housing using welded construction. Thearmature is normally made from a 50% nickel-iron alloy and brazed to astainless steel extension. Secondary leads are usually shielded tominimize channel-to-channel crosstalk for multi-channel units and toshield components from RF energy.

The length and diameter of an LVDT must be sufficient to allow adequatewinding space for achieving the desired electrical performance, supportany pressure requirement and withstand the environmental shock,vibration and acceleration. Where physical size is limited, electricalperformance must be flexible. Although the LVDT is basically a simpledevice, the operating characteristics and electrical parameters arecomplex and depend to a large extent on the physical limitations.

U.S. patent application Ser. No. 09/547,511, filed Apr. 12, 2000, whichis assigned to the assignee of this application, discusses some of theparameters that designers consider when specifying the sizes of LVDTcomponents. That discussion and the remained of the application areincorporated by reference.

An LVDT's output voltage is proportional to the voltage applied to theprimary. System accuracy depends on providing a constant input to theprimary or compensating for variations of the input by using ratiotechniques. The output can be taken as the differential voltage or, witha center tap, as two separate secondary voltages whose difference is afunction of the displacement. If the sum of the secondary voltages isdesigned to be a specific ratio of the difference voltage, overallaccuracy significantly improves.

Most LVDTs deal with a linear mechanical input in that a force acts on aprobe that directly or indirectly carries the armature of the LVDT. Theforce moves the probe longitudinally along the axis of the probe. Thedevice described in Ser. No. 09/547,511 is such a device. The output ofsome systems is rotational instead of linear. Consequently, there is aneed for transducers that can measure the extent of a rotation. Rotaryvariable differential transducers (RVDT) having a circular geometry areknown. They have been employed to measure the angular position of arotary shaft. Known rotary variable differential transformers are moredifficult and expensive to manufacture than LVDTs, however.

In some instances, tying the rotational output to a single LVDT producesacceptable results. However, many systems benefit with increasedaccuracy from multiple LVDTs.

INVENTION SUMMARY

Providing an LVDT-based transducer that measures rotation is a principalobject of the present invention. Designing such a system that isaccurate and reliable is a related object. Another object is providingredundancy in such a transducer. These and other object are evident forthe following description of the present invention.

The linear variable differential transformer (LVDT) system of thepresent invention comprises a rotary input. The input connects to a gearreduction system or rotary head, which produces a reduced, increased, orunchanged rotary output in an output member. The output member isthreaded, and a nut is threaded onto the output member and translates asthe output member rotates. The nut is restrained against angularrotation. The armatures of one or more LVDTs attach to the nut. Eacharmature assembly includes a magnetic member. Consequently, as the inputrotates and the gear reduction causes the output member to rotate moreslowly, the nut translates along the threaded output member. Themovement of the armature assemblies moves the magnetic members to createa differential voltage output from the secondary coils of the LVDT.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view, principally in section, of the rotary to linearLVDT system of the present invention.

FIG. 2 is a sectional view taken through plane 2-2 of FIG. 1 of theexemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The LVDT system of the present invention attaches to a rotary input 10.The rotary input can be the rotary output of any device that rotatesthrough a limited range of revolutions. For example, some positioningdevices have shafts that rotate as position changes. One can determinethe change of position of such a device by determining the extent ofrotation of a shaft

In the exemplary embodiment, the rotary output is a shaft 20 thatextends from a gear reduction system 30 to a fixed support member 24.The fixed support member mounts within rear housing member 60. The rearhousing may have a shoulder 25 to locate the fixed support member. Therear housing member attaches to the front housing member 62.

Rear housing member 60 has a radial flange 64 that butts up againstradial flange 66 of front housing member 62. The front housing sectionalso has an annular flange 68, which extends under the radial flange ofthe rear housing member. An O-ring 70 seals the two housing members. Theradial flanges 66 and 64 are welded together at the joint 74.

Rear housing member 60 also has a rear section 90. Electric wires (notshown) extend out of section 90. The wires feed an A/C signal to thecoil assemblies of the LVDT as discussed below.

The gear reduction system 30 between the input 10 and the rotary output20 transfers the rotary input into rotation in the rotary output and mayreduce the angular rotation of the rotary output relative to the angularrotation of the rotary input. Gear reduction systems are common and caninclude gears, pulleys or other simple machines for reducing orincreasing an output relative to a rotating input.

As FIG. 1 shows, rotary output member 20 has a narrow diameter end 22.That end is supported within journaled housing 24. The journaled housinghas bearings 26 or other friction-reducing means which support and allowthe end 22 of rotary output member 20 to rotate freely. Support 24 mayhave multiple openings, only two, openings 28 and 29 are shown inFIG. 1. LVDT extensions 102, 104, 106 and 108 (FIG. 2) extend throughthe openings in the support housing. The openings are sufficiently largeto allow the extensions to move longitudinally relative to the openings.The exemplary embodiment has four armatures, but the system wouldfunction with one, two, three, four or more armatures.

A coil assembly surrounds each armature. FIG. 1 shows two coilassemblies 110 and 112 surrounding extensions 102 and 108. The coilassemblies have a primary coil and two secondary coils as shown in Ser.No. 09/547,511. FIG. 1 does not show the separate coils of the coilassemblies. The coil assemblies are fixed to support member 24. Becausethe support member remains stationary, the coil assemblies also do notmove.

Rotary output member 20 is threaded, as FIG. 1 shows in part. Nut 120(FIGS. 1 and 2) threads onto the threads of rotary output member 20. Theinside of opening 122 of nut 120 is threaded to match the threads on therotary output member 20. Thus, as the rotary output member 20 rotates,nut 120 translates along the rotary output member. Nut 120 has fouropenings 110, 112, 114 and 116 (FIG. 2) that receive the ends of therespective extensions 102, 104, 106 and 108. Fasteners may be providedto secure the armatures into the openings in the nut. Small channels 118(FIG. 1) provide access for a mechanical fastener such as a set screw toattach the armature to the nut.

The nut may also have channels 124 and 126 for receiving guides (notshown) on the inside of the front or rear housing members 62 and 60 toprevent the nut 120 from rotating. In the exemplary embodiment, nut 120may not translate into the rear housing 60. Therefore, any guide onlywould be mounted within front housing member 62. Each armature has amagnetic member 109 (FIG. 1). As output member 20 rotates and nut 120translates relative to the output member, the four armatures alsotranslate relative to support member 24. The direction of translationdepends on the direction of rotation of rotary output member 20 and, ofcourse, rotary input member 10. Translation of the magnetic members 109within the extensions 102, 104, 106 and 108 moves the magnetic membercloser to one secondary coil and farther from the other secondary coilin each coil assembly. As discussed in detail in application Ser. No.09/547,511, the coils in the coil assemblies receive alternatingcurrent. As each armatures moves relative to the two secondary coils,the magnetic member within the aperture changes the inductance in thecoils. That causes a voltage change in each coil assembly, and thatvoltage differential between the secondary coils is read by anelectrical device attached at fitting 92.

Several components of the present invention contribute to systemaccuracy. First, there are four armatures and four coil assemblies. Anyinaccuracies tend to balance out among the four sub-systems. Second, thefixed connections of the extensions 102, 104, 106 and 108 to nut 120maintains the nut perpendicular to the rotary output member 20. Thistends to minimize backlash and maximizes accuracy. Passing the armaturesthrough openings 28 and 29 in fixed support 24. Because the armaturesare kept straight, they, in turn, keep the nuts 120 straight.

While the specification describes particular embodiments of the presentinvention, those of ordinary skill can devise variations of the presentinvention without departing from the inventive concept.

1. A linear variable differential transformer (LVDT) system comprising:a) a rotary input; b) a rotary output; c) a gear reduction systembetween the rotary input and the rotary output for transferring therotary input into rotation of the rotary output and reducing the angularrotation of the rotary output relative to the angular rotation of therotary input; d) one or a multiple of LVTDs, each LVDT comprising aninternal magnetic member and a coil assembly, the coil assemblycomprising a primary coil between two secondary coils, an alternatingcurrent in the coil assembly, the LVDT measuring the differentialvoltage between the two secondary coils as the magnetic member movesrelative to the two secondary coils; and e) a translating membertranslating on the rotary output and being operably connected to themagnetic members of the LVDTs.
 2. A linear variable differentialtransformer (LVDT) system comprising: a) one or a multiple of LVTDs,each LVDT comprising an internal armature and a coil assembly, the coilassembly comprising a primary coil between two secondary coils; b) athreaded nut attached to the armatures; c) a rotating member having atleast a portion threaded, the threaded nut being threaded to thethreaded portion of the rotating member, whereby rotation of therotating member translates the nut to move the armatures relative to thecoil assemblies;
 3. The LVDT system of claim 2 wherein the rotatingmember comprises a rotating input member and a rotating output member,the threaded portion of the rotating member being around the rotatingoutput member, a gear reduction system between the rotary input memberand the rotary output member for transferring the rotation of therotating input member into rotation in the rotary output member andreducing the angular rotation of the rotary output member relative tothe angular rotation of the rotary input member, whereby rotation of therotary output member translates the nut.
 4. The LVDT system of claim 2further comprising a support member spaced from the nut, the supportmember having openings for receiving the armatures.
 5. The LVDT systemof claim 2 further comprising a support member spaced from the nut andreceiving an end of the rotary output member.
 6. The LVDT system ofclaim 5 wherein the support member has openings for receiving thearmatures.
 7. A method of measuring the amount of rotation in a rotatingmember comprising: a) threading a nut on the rotating member whereby thenut translates along the rotating member as the rotating member rotates;b) driving an armature of at least two LVTDs relative to a coil assemblyof each LVDT, each coil assembly comprising a primary coil and twosecondary coils, the armatures being operably connected to the nutwhereby translation of the nut drives the armatures; c) comparing thevoltage differences between the two secondary coils of each coilassembly.
 8. The method of claim 7 wherein the rotating member comprisesa rotating input member and a rotating output member, the threadedportion of the rotating member being around the rotating output member,the method further comprising reducing the angular rotation of therotating output member relative to the rotating input member.
 9. Themethod of claim 7 wherein the rotating member comprises a rotating inputmember and a rotating output member, the method further comprisesproviding a gear reduction assembly intercoupling the input member andthe output member.